Question: What is the greatest common factor of $40$ and $15$ ? Another way to say this is: $\operatorname{gcf}(40, 15) = {?}$
Answer: The greatest common factor is the largest number that is a factor of both $40$ and $15$ The factors of $40$ are $1$ $2$ $4$ $5$ $8$ $10$ $20$ , and $40$ The factors of $15$ are $1$ $3$ $5$ , and $15$ Thus, the greatest common factor of $40$ and $15$ is $5$. $\operatorname{gcf}(40, 15) = 5$